1. Field of the Invention
The invention relates to catalytic cracking and reforming.
2. Description of Related Art
The present invention relates to an unusual way of overcoming a problem in one mature processes, catalytic reforming by making unconventional use of another mature process, catalytic cracking. The problem is low octane number and/or excessive benzene content in the C6 reformate.
Catalytic reforming of naphtha boiling range feeds over platinum based catalyst to produce high octane reformate is one of the most successful processes in the world. More than a hundred units are in use, converting low octane naphthas to high octane, aromatic rich gasoline. The only problem with the process is that the product inherently contains large amounts of aromatics, including benzene. Many localities are limiting the amount of benzene which can be contained in gasoline, because of the toxic nature of benzene. Another minor problem in some catalytic reforming units is that the octane number of the gasoline produced varies significantly with boiling range. The light reformate, e.g, the C6- fraction, sometimes has a lower octane than desired and lower than the octane of the C7+ fraction. The C6- fraction can be doubly troubling to refiners, having a shortage of octane and an excess of benzene.
Usually refiners have looked on catalytic reformers as aromatics generators, and welcomed, rather than avoided, the production of aromatic hydrocarbons. Many refineries recover benzene rich streams, usually by L/L extraction with a solvent such as Sulfolane, and then alkylate the purified streams with light olefins. Such processing, to produce ethylbenzene or xylenes, usually requires highly purified benzene. This approach inherently tends to produce a by-product gasoline with a lower benzene content, but can only be justified when there is demand for a high value product such as ethylbenzene, and capital available to build expensive solvent purification facilities. There is a need for a simpler approach, which can make better use of existing refinery facilities, and which requires only fractionation to isolate a benzene rich fraction.
Refiners considering benzene in light reformate a problem, rather than a source of valuable petrochemicals, have solved the problem in various ways. In U.S. Pat. No. 4,140,622, Herout et al, which is incorporated herein by reference, a reformate was fractionated to provide a benzene rich fraction which was then mixed with C3/C4 olefins and passed over an alkylation catalyst such as SPA, or solid phosphoric acid. In U.S. Pat. No. 4,209,383, Herout et al, which is incorporated herein by reference, a reformate and C3-C4 olefins from an FCC were combined, then passed through a dehexanizer, then passed through an alkylation zone.
While concentrating a benzene rich fraction by distillation with light olefins will work, it requires a source of light olefins. Most refineries with catalytic cracking units produce large amounts of light olefins, but invariably also have HF or sulfuric acid alkylation units which convert these olefins into high octane, aromatic free, alkylate. Thus once a refiner puts in a cat cracker, it is usually essential to put in an "alky" unit to consume the olefins generated by the cat cracking process, and conversion of light olefins to alkyl aromatics (by reaction with a benzene rich fraction of reformate) reduces the amount of non-aromatic gasoline that can be produced by alkylation, and increases the production of aromatic gasoline, although the aromatics will be heavier than benzene by virtue of alkylation. This can be better understood by reviewing briefly what goes on during conventional catalytic cracking.
Catalytic cracking of hydrocarbons has enjoyed worldwide success. It is probably the method of choice for converting a heavy feed into lighter, more valuable products, the most valuable of which are usually the high octane gasoline and the light olefins. Catalytic cracking of hydrocarbons is carried out in the absence of externally supplied H2, in contrast to hydrocracking, in which H2 is added during the cracking step. An inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In the fluidized catalytic cracking (FCC) process, hydrocarbon feed contacts catalyst in a reactor at 425C-600C, usually 460C-560C. The hydrocarbons crack, and deposit carbonaceous hydrocarbons or coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then regenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Coke burns off, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 500C-900C, usually 600C-750C.
Modern FCC regenerators tend to operate at fairly high temperatures, both to minimize CO emissions and as a reflection of the heavier feeds now being processed in FCC units. Most FCC units operate in "heat balanced" operation, with the heat energy needed to crack fresh feed being supplied by burning coke deposited on the catalyst during the cracking reaction. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is discharged into the atmosphere.
Cracked products are fractionated into light, olefin rich gas, gasoline, light and heavy cycle oils, and slurry oils. The olefinic light gasses are usually alkylated with isobutane in the presence of sulfuric or HF acid, to produce high octane alkylate which is essentially free of aromatics. The cycle oils are valuable as fuel, and relatively refractory to further processing in the FCC. Many units recycle modest amounts of heavy material, sometimes heavy cycle oil or more likely slurry oil, the heaviest product. These materials, especially slurry oil, are very difficult to crack further, and much of the recycled material is converted to coke in the FCC, and some lighter product.
Most older FCC units regenerate the spent catalyst in a single dense phase fluidized bed of catalyst. Although there are myriad individual variations, typical designs are shown in U.S. Pat. No. 3,849,291 (Owen) and U.S. Pat. No. 3,894,934 (Owen et al), and U.S. Pat. No. 4,368,114 (Chester et at.) which are incorporated herein by reference.
Most new units are of the High Efficiency Regenerator (H.E.R.) design using a coke combustor, a dilute phase transport riser, and a second dense bed, with recycle of some hot, regenerated catalyst from the second dense bed to the coke combustor. Units of this type are shown in U.S. Pat. No. 3,926,778 (which is incorporated by reference) and many other recent patents. The H.E.R. design is used in most new units because it permits operation with less catalyst inventory (and hence less catalyst loss), and because such units tend to emit less CO and less NOx than the single dense bed regenerators.
In general, there has not been much integration of catalytic reforming and cat cracking. Catalytic reformate is usually considered a high octane product which does not need further upgrading, nor sulfur removal. In contrast, catalytically cracked gasoline, while usually of high octane, may require some further processing to remove sulfur compounds. So far as is known, no refiner has ever added reformate to an FCC, other than perhaps as a way of getting rid of off spec product or disposing of slop hydrocarbon streams. Although no integration of FCC and reforming per se has occurred, some work has been reported on fluidized bed catalytic processing of benzene rich streams.
Owen, in U.S. Pat. No. 3,969,426, which is incorporated herein by reference, reported that purified aromatic streams (benzene and toluene) could be used to convert durene, in a fluid bed reaction which generated almost no coke. Catalyst was regenerated intermittently, and preferably at a regeneration temperature of about 1000.degree. to 1050.degree. F. Although these conditions do not mesh well with conventional catalytic cracking operation, wherein coke deposition is needed for heat balanced operation, and regenerator temperatures are usually around 1200.degree.-1350.degree. F., the work was of interest because it showed that a mixture of durene, benzene and toluene could be converted in a bench scale riser reactor to a substantially durene-free, high quality gasoline product with only a trace loss of carbon to gas or coke. The feed was a mixture of durene (20 wt %) benzene (20 wt %) and toluene (60 wt %). The riser reactor used clean burned, 15 wt % REY zeolite catalyst having a 67.5 FAI. The riser reactor inlet mix temperature was 800.degree. F. and 900.degree. F. and the cat:oil ratio was 10.12. Essentially complete aromatic carbon retention was achieved, with less than 1 wt % of the feed going to coke, and about 0.5 wt % going to gas. Durene levels were reduced from 20 wt % to 0.2-0.4 wt %. Benzene levels were reduced from 20.0 wt % (feed) to 16.64 to 16.95 wt % (product).
Although this reduced the benzene content, it required the addition of durene to the gasoline boiling range material. The durene, if not almost completely eliminated, will appear in the gasoline product and cause problems because of its high melting point. Durene is not readily available in most refineries, it is primarily a by-product of methanol to gasoline processing.
Conversion of heavy reformate, or a pyrolysis naphtha having an IBP of 230-250 and EP of 350.degree.-430.degree. F., into benzene in a fluidized bed unit was reported in U.S. Pat. No. 4,066,531, Owen et al, which is incorporated herein by reference. A heavy reformate was reacted over a porous acid-active zeolite catalyst having a fluid activity index of at least 18, in the absence of added hydrogen, at 800.degree. to 1200.degree. F. in a fluidized system (a riser reactor is shown) having a catalyst residence time of 0.1 to 20 seconds. This would tend to increase the benzene content of the gasoline pool, because there is a net production of benzene.
To summarize, there is no way to effectively deal with the problem of too much benzene in reformate with known technology. Aromatics extraction will work, but costs too much. Aromatics alkylation with light olefins, even processes using relatively dilute benzene streams still requires a separate alkylation reactor, and consumes light olefins that could be converted into nonaromatic alkylate. Attempts to increase gasoline production in refineries having cat cracking units by recycling heavy cycle oil or slurry oil to the cracker will achieve only modest increases in gasoline production, and increase the amount of coke that must be burned in regenerators that are being pushed to their metallurgical limits in many instances.
We discovered a way to take a light reformate stream, and process this stream in a conventional catalytic cracking unit to reduce the benzene content of the reformate. We can convert benzene without isolating it from non-aromatics. The process works to reduce the benzene content of reformate even when essentially no changes are made to operation of the FCC, other than addition of a benzene containing reformate. In preferred embodiments, we increase the efficiency of benzene reduction in the fluidized catalytic cracking unit by modifying the unit, or adding the reformate in an unusual way, and using various feed and catalyst additives.